FOX considers the box-office power of the cast of the original trilogy. The reason why they brought back the original cast is to gain money. They saw how big the Avengers was and now they mixing the cast of the original cast and First Class. I see the logic of FOX trying to sell the First Class cast by teaming them up with the cast of the OT. If this movie ends very well at the box-office because of the returning cast of the original trilogy, FOX might just want to move with the original cast and stop making movies with the cast of First Class. Plus with movies set in the past, they would keep re-introducing characters. But with the present timeline, with the cast of original trilogy, they could do so much more.

FOX doesn't need to "sell" the First Class cast. Not with the actors they have leading it. I don't see FOX dropping the FC series in favor of the decade old originals. IMO post DOFP will be FC or FC and X4. They have JL, JM, and MF locked up for a 3rd film. You think FOX is throwing that away?